1. Field of the Invention
The present invention relates to power transmission lines, and particularly to a method of converting high voltage AC lines into bipolar high voltage DC systems.
2. Description of the Related Art
There is growing interest in the development of renewable energy sources, such as wind power. Many wind power stations are located offshore and transmit power onshore through submerged high voltage DC lines. However, for historical reasons, existing onshore power transmission lines are based on three-phase high voltage AC transmission line systems. In order to take maximum advantage of the existing power transmission infrastructure while reducing costs, it would be desirable to convert existing AC power transmission lines for use as high voltage DC transmission lines.
Current approaches to high voltage DC transmission lines include a tripole HVDC system, which consists of thyristor-based or modular multilevel converter-based systems. This type is suitable only in point-to-point transmission applications. On the other hand, three-wire bipole structures (TWBS) use a power electronics-based converter fed from an isolated AC system to achieve the required performance. In this approach, the converter is connected in series with one of the wires (the modulating wire). This affects system reliability and adds more complexity.
Thus, a method of converting high voltage AC lines into bipolar high voltage DC systems solving the aforementioned problems is desired.